This invention is related to master cylinders that are adapted to be pre-filled and pre-bled before being taken to an assembly site such as an automotive plant. Typically, such master cylinders comprise an elongated, hollow, cast metallic body defining an internal pressure chamber and including a pressure producing piston. A cap on one end of the body closes the chamber and has an opening for the piston rod. The pressure chamber is connected to an integral or remote reservoir for replenishing any fluid loss in the system. The reservoir has a filling opening, and a vented cap is mounted on the filling opening. Prefilled master cylinders of this general construction are shown for example in U.S. Pat. No. 4,407,125 entitled "Liquid Pressure Apparatus," issued on Oct. 4, 1983 to David Parsons and assigned to Automotive Products plc of Warwichsire, England.
Master cylinders of this general construction are generally satisfactory but are relatively expensive and complicated to produce. Further, there is a problem with maintaining master cylinders of this type since frequently a mechanic will remove the cap of the reservoir to replenish the hydraulic fluid but fail to remove the bellows positioned within the open upper end of the reservoir with the result that hydraulic fluid is introduced into the bellows rather than into the working portion of the reservoir.